The invention relates to a transmission where clutches or brakes are closed by spring force and hydraulically reopened in order to connect the input of the transmission with the output of the transmission, or to separate or shift different ratio steps.
The transmission of the above described, kind is often a reversing transmission such as used in lift trucks like loaders. One clutch for a forward gear and one clutch for a reverse gear is preferably used, but it can also be the input assembly of a multi-gear reversing transmission especially used in construction machinery like loaders, excavators, etc.
Transmissions of the kind which used clutches or brakes closed by spring force and reopened by a hydraulic force have the advantage that by modulation of the actuating pressure, the clutches or brakes can be shifted so that a controlled load take-over results. This is primarily needed in the reversing operation of a lift truck or of an emergency deceleration via a multi-disk clutch or a multi-disk brake until stoppage, since otherwise the result would be an inadmissibly high or too slow a deceleration.
DE 195 17 888 A1 discloses a reversing transmission shiftable under load in which the reversing clutches are closed by spring force and hydraulically reopened and a hydraulically opened clutch can be shifted via a valve for closing, the line to the actuating device of the clutch being connected with an accumulator and a diaphragm. The volume of the accumulator is designed substantially large enough to be able to contain the actuating device so that the disks can come into contact quickly after triggering of the gear shift. It is thus possible to bring a previously opened clutch quickly to a position in which the disks come into contact, since the volume in the actuating device is supplied to the accumulator. Due to elastic deformations of the parts, the disks only come into contact when this residual pressure fluid, when the accumulator is filled, is removed from the actuating device of the clutch via a throttle point in the pressure medium reservoir. After the disks have come into contact and the accumulator has been filled with this oil volume, a pressure-modulated load take-over of the clutch takes place by the oil of the accumulator flowing off into the pressure medium reservoir via a diaphragm and thus a pressure which is determined by the accumulator and likewise crops up on the actuating device of the clutch, is still maintained over a specific period of time. If a previously hydraulically opened clutch is shifted, via a valve for closing, the accumulator, which is connected with the clutch, is filled with oil and slowly discharges via the diaphragm toward the pressure medium reservoir. If a gear shift again takes place in which the actuating device of the clutch is again loaded with pressure and the accumulator still has not completely discharged its oil volume, via the diaphragm, into the pressure medium reservoir, then the accumulator is partly filled. If then one other gear shift occurs in which the clutch again has been actuated for closing and thus the actuating device of the clutch again becomes connected with the accumulator, it is not possible for the accumulator to contain the complete oil volume of the actuating device of the clutch, since pressure fluid of the preceding gear shift still exists in the accumulator and thereby the disks of the clutch do not immediately come into contact, since the pressure fluid in the actuating device of the clutch must now discharge via the diaphragm. The load take-over of the clutch occurs with a time delay so that, e.g. in case of a desired downshift, the vehicle still covers an inadmissible distance until a load take-over. If the function of the parking brake or the dynamic emergency brake takes place, via two reciprocally stressing clutches, there can thus be produced, during such previously indicated closely consecutively following gear shifts, a condition in which the driver has introduced the parking brake by the load take-over of the clutch occurs only thereafter which could lead to an inadmissible and dangerous situation.
The problem on which this invention is based is to provide a transmission having a clutch system in which at least one clutch is closed by spring force and can be hydraulically reopened, the load take-over of the clutch proceeds under a pressure modulation and the time until load take-over of the clutch is independent of the distance in time of the gear shifts.
This problem is solved with a transmission according to the preamble and also including the characteristic features of the main claim.
According to the invention, the clutch or brake is opened by the actuating device of the clutch or the brake being loaded with pressure while, with the aid of a valve, the actuating device of the clutch or the brake is connected with a source of pressure medium. If the clutch or the brake is closed, the actuating device of the clutch or the brake is connected with the aid of a valve with an accumulator and a diaphragm so that a spring, which acts upon the actuating device of the clutch or the e, moves the oil volume in the actuating device into the accumulator thus bringing the disks of the clutch or the brake into contact. The volume of the accumulator is designed in a manner such as preferably to correspond to the volume of the actuating device of the clutch or the brake. If a certain pressure level has been reached in the actuating device or the accumulator, the valve, which is connected with the actuating device or the accumulator, opens so that the pressure medium can flow out from the actuating device directly into the pressure medium reservoir. The valve automatically closes again when a certain pressure level has been reached in the actuating device or the accumulator. It is thus ensured that regardless of whether or not the accumulator has already been filled with pressure medium the initial situation for the pressure-modulated take-over in the clutch is always the same, since above a certain pressure level the pressure medium reservoir until at a previously defined pressure level the accumulator is entirely filled with pressure medium and the disks of the clutch come into contact. The volume of the accumulator can thus be designed also independently of the volume of the actuating device of the clutch or the brake, since the accumulator no longer must absorb all the volume of the actuating device inasmuch as the pressure-modulated load take-over of the clutch depends only on the pressure-time curve which depends on the design of the accumulator and the diaphragm with which the accumulator is connected and through which the pressure medium of the accumulator flows into the pressure, medium reservoir. The accumulator is preferably configured so that, via the piston of the accumulator, a communication to the pressure medium reservoir always results when the accumulator is completely filled with pressure medium. However, the communication of the accumulator or the actuating device of the clutch with the pressure medium reservoir above a certain pressure level can also exist via a separate valve in which case additional parts are needed. For the pressure-modulated load take-over, the volume of the accumulator discharges via a diaphragm into the pressure medium reservoir. The diaphragm can be designed as an independent part, but it is also convenient to design the annular gap which forms between the piston of the accumulator and the cylinder wall thereof so, that a defined throttle point can be used as drainage of the pressure medium from the accumulator of the pressure medium reservoir. While the accumulator no longer has to absorb the whole volume of the actuating device but, above a certain pressure level, removes an excessive volume directly into the pressure medium reservoir, the previously defined pressure level can be designed so that immediately after the gear shift for closing of the clutch the disk set abuts and transmits a defined torque. Thus any time delay up to load take-over is eliminated, since also the pressure medium volume which as a consequence of elastic deformation of the actuating device and of the clutch, still has to be removed until load take-over likewise is removed down to a previously defined pressure level directly into the pressure medium container. The inadmissible time delay during load take-over of a parking brake, or an emergency brake, or a reversing clutch is thus entirely eliminated.